A DC-DC converter circuit, which is used as a power supply circuit for driving a CPU (Central Processing Unit) used in, for example, a personal computer or a server, has been required to have a low-voltage, large-current drive capability as well as a small size and a low power consumption. It is necessary that a coil incorporated in the power supply circuit is made smaller in order to achieve the downsizing of the power supply circuit. For its achievement, the increase of the PWM (Pulse Width Modulation) frequency is inevitably required. Also, it is necessary to reduce parasitic capacitance and on-resistance of a power MOSFET serving as a switching element of the power supply circuit in order to achieve the low power consumption. A principal portion of this power supply circuit is provided with a power MOSFET for a switching element and a power MOSFET for a rectifier. For the present, the PWM frequency is in a range of about 200 kHz to 500 kHz. Therefore, power MOSFETs having a vertical structure advantageous to low on-resistance are used as both of the power MOSFETs for the switching element and the rectifier.
Note that the power supply circuit is disclosed in, for example, IEEE “0.35 μm, 43 μΩcm2, 6 mΩ Power MOSFET to Future Microprocessor” in 1999 (Virginia University), Electronic Design Dec. 6, 1999 “MOSFET selection is Key to successful DC-DC conversion”, and “Device Requirements for Future cpu Voltage Regulators” Intel Corporation.